1. Field of the Invention
The present invention relates to a power supply control device and method. More particularly, the invention relates to a power supply control device having a semiconductor switch which is controlled in accordance with a control signal supplied to a control signal input terminal to control the supply of electric power from a power supply to a load under the switching control, and a power supply control method in use for the power supply control device.
2. Related Art
The power supply control device with a semiconductor device, which arranged as shown in FIG. 8 is known. This power supply control device, which is used in a vehicle, selectively supplies electric power source from a battery to respective loads, and controls the power supply to the load.
As shown, the related power supply control device is arranged such that a shunt resistor RS and a drain (D)xe2x80x94source (S) path of a thermal FET QF are connected in series in a power supply path for supplying an output voltage VB of a power supply 101 to a load 102 including headlights, power windows and others. The power supply control device includes a driver 901 which detects a current flowing through the shunt resistor RS and controls the drive of the thermal FET QF by a hardware circuit, an A/D converter 902 for translating the analogue value of a current monitored by the driver 901 into the digital value, and a microcomputer (CPU) 903.
The thermal FET QF is contained with a thermosensor (not shown). The thermal FET QF as a semiconductor switch has an overheat shut-off function to forcibly shut off the thermal FET QF by a gate shut-off circuit contained therein when temperature of the thermal FET QF rises in excess of a predetermined temperature. In the figure, RG is a resistor, and ZD1 is a Zener diode which keeps the voltage between the gate G and the source S of the thermal FET QF at 12V, and when overvoltage is applied to the gate G of the FET, forms a bypass for a current caused by the overvoltage.
This related power supply control device has a protecting function against an over current flowing through the load 102 or the drain-source path of the thermal FET QF. The driver 901 includes differential amplifiers 911 and 913, which serves as a current monitoring circuit, a differential amplifier 912 as a current restricting circuit, a charge/pump circuit 915, and a drive circuit 914 which drives the gate G of the thermal FET QF through the resistor RG based on an on/off control signal from the microcomputer (referred to as MICROCOMPUTER) 903 and the overcurrent judgement result output from the current restricting circuit.
When the differential amplifier 912 judges on a voltage drop through the shunt resistor RS that the current exceeds a reference value (upper limit), viz., it detects overcurrent, the drive circuit 914 turns off the thermal FET QF. Thereafter, when the current decreases to below another reference value (lower limit), it turns on the FET again.
The MICROCOMPUTER 903 monitors the current by the current monitor circuit (differential amplifiers 911 and 913) at all times. When an abnormal current in excess of a normal value flows, the MICROCOMPUTER 903 stops the drive current for the thermal FET QF to turn off the FET. When temperature of the thermal FET QF exceeds a prescribed value before a drive signal for the off control is output from the MICROCOMPUTER 903, the overheat shut-off function operates to turn off the thermal FET QF.
The related power supply control device mentioned above needs the shunt resistor RS connected in series to the power supply path, in order to make the current detection. In the recent trend of large load currents, which induces reduction of the resistance of the thermal FET QF, the heat loss by the shut resistor is not negligible.
The overheat shut-off function and the overcurrent restricting circuit operate when an almost perfect short occurs in the load 102 or the wiring, and large current flows. However, those fail to operate when a layer short, e.g., an imperfect short having some short resistance, occurs and small short current flows. A possible measure taken for the layer short is that the MICROCOMPUTER 903 detects an abnormal current by only using the monitor circuit and turns off the thermal FET QF. A response of the MICROCOMPUTER basis control to such an abnormal current is poor.
Since the shunt resistor RS, the MICROCOMPUTER 903 and the like are indispensably used in the related power supply control device, a large packaging space is required. Additionally, those part and component are relatively expensive, so that the resultant device is high in cost.
Accordingly, the present invention is directed to solve the above problems and to eliminate the inconvenient circumstances, and has an object to provide a power supply control device and method which eliminates the shunt resistor connected in series to the power supply path in order to detect the current, thereby repressing the heat loss, quickly responses to an abnormal current flowing when a layer short, e.g., an imperfect short having some resistance, occurs, and is easy to integrate and is low in cost.
According to the present invention, there is provided a power supply control device comprising: a semiconductor switch being controlled in accordance with a control signal supplied to a control signal input terminal to control the supply of electric power from a power supply to a load under the switching control; a reference current generating device for generating a reference current; a reference voltage generating device for generating a reference voltage based on the reference current; a detector for detecting a difference between a voltage between terminals of the semiconductor switch and the reference voltage; and a controller for on/off controlling the semiconductor switch in accordance with a detected difference between the terminal-terminal voltage and the reference voltage.
In the power supply control device, the reference voltage generating device includes a second semiconductor switch controlled in accordance with the control signal, and the second semiconductor is connected in series to the reference current generating device to form a series circuit, which is connected in parallel to the semiconductor switch and the load, whereby the terminal-terminal voltage of the second semiconductor switch is generated in the form of the reference voltage.
In the power supply control device, the reference current generating device generates a reference current dependent on an output voltage of the power supply.
In the power supply control device, the reference current generating device is a constant current source.
In the power supply control device, a voltage characteristic of the reference voltage of the reference voltage generating device is substantially equal to a voltage characteristic in a state that a target current flows, which is in excess of a maximum current within a range where the semiconductor switch and the load are normally operable.
In the power supply control device, the semiconductor switch and the second semiconductor switch are equivalent to each other in a transient voltage characteristic of the terminal-terminal voltage when each semiconductor switch shifts its state from an off state to an on state.
The power supply control device further includes overheat protector operating such that when the semiconductor switch is overheated, the overheat protector turns off the semiconductor switch, to thereby protect the semiconductor switch.
In the power supply control device, the semiconductor switch, the reference voltage generating device, the detector, the controller, and the overheat protector are formed on a same chip.
In the power supply control device, the reference current generating device is located outside the chip.
The power supply control device further comprises prohibiting device for prohibiting the controller from turning on and off the semiconductor switch for a fixed period after the semiconductor switch is turned on.
The power supply control device further includes number-of-times controller operating such that the number-of-times controller adds up the number of times the controller turns on and off the semiconductor switch, and when the number of on/off control times reaches a predetermined number of times, the number-of-times controller turns off the semiconductor switch.
According to the present invention, there is provided a power supply control method in use for a power supply control device including a semiconductor switch which is controlled in accordance with a control signal supplied to a control signal input terminal to control the supply of electric power from a power supply to a load under the switching control, the power supply control method comprising the steps of: generating a reference current; generating a reference voltage based on the reference current; detecting a difference between terminals of the semiconductor switch and the reference voltage; and on/off controlling the semiconductor switch in accordance with a detected difference between the terminal-terminal voltage and the reference voltage.
In the power supply control devices and in the power supply control method, when the supply of electric power from the power supply to the load is controlled by the semiconductor switch in a switching manner, a reference current is generated by the reference current generating device (reference current generating step). A reference voltage is generated based on the reference current by the reference voltage generating device (reference voltage generating step). A difference between the voltage between terminals of the semiconductor switch and the reference voltage is detected by the detector (detecting step) The semiconductor switch is on/off controlled in accordance with the detected voltage difference between the terminal-terminal voltage and the reference voltage by the controller (control step).
The semiconductor switch (second and third semiconductor switches to be described later) may be any of the following switching elements: field effect transistors (FETs) and static induced transistor (SITs), or emitter switched thyristors (ESTs), MOS complex devices, e.g., MOS controlled thyristors (MCTs), and insulated gate power devices, e.g., IGBT (insulated gate bipolar transistors. Those switching elements may be of the n-channel type or the p-channel type.
In particular, in the power supply control device, it is preferable that the reference voltage generating device includes a second semiconductor switch controlled in accordance with the control signal. Further, the second semiconductor is connected in series to the reference current generating device to form a series circuit. The series circuit is connected in parallel to the semiconductor switch and the load. The terminal-terminal voltage of the second semiconductor switch is generated in the form of the reference voltage. It is desirable that the reference current generating device, as in the third power supply control device, generates a reference current dependent on an output voltage of the power supply, or that it is a constant current source, as in the fourth power supply control device.
In the power supply control device, it is desirable that a voltage characteristic of the reference voltage of the reference voltage generating device is substantially equal to a voltage characteristic in a state that a target current flows, which is in excess of a maximum current within a range where the semiconductor switch and the load are normally operable. In the sixth power supply control device, it is desirable that the semiconductor switch and the second semiconductor switch are equivalent to each other in a transient voltage characteristic of the terminal-terminal voltage when each semiconductor switch shifts its state from an off state to an on state.
In a case where an FET is used for the semiconductor switch, the terminal-terminal voltage (between the drain and the source) of the FET forming a part of the power supply path varies depending on states of the power supply path and the load, viz., a time constant defined by wiring inductance and resistance of the path and short resistance, in the voltage characteristic when the FET shifts its state from an on state to an off state (the descending voltage characteristic in the case of the n-channel FET). In a normal operation where no short occurs, the terminal-terminal voltage swiftly converges to a voltage value below a reference voltage value. Where a short occurs, it does not converge to below the reference voltage value. Where an imperfect short having some resistance occurs, it takes a long time till the terminal-terminal voltage converges to below the reference voltage value.
The present invention utilizes the transient voltage characteristic of the semiconductor switch when it shifts its state from an off state to an on state. In the invention, judgement is made as to whether or not the terminal-terminal voltage of the semiconductor switch (or the current in the power supply path) , which forms a part of the power supply path, is out of a normal state by detecting a difference between the terminal-terminal voltage of the semiconductor switch and the reference voltage generating device (reference voltage generating step). Overcurrent can be detected by the detector (detecting step) if the voltage characteristic of the reference voltage is set to bear a closest resemblance to the voltage characteristic in a state that a target current, which is in excess of the maximum current within a normal operation range, flows to the load.
Therefore, the power supply control device and method of the invention does not need the shunt resistor, conventionally used, which is connected in series to the power supply path for the current detection. With this feature, the heat loss of the device is suppressed. An abnormal current by a layer short, such as an imperfect short having some short resistance, as well as an overcurrent by a perfect short, can continuously be detected by use of the hardware circuit or the program processing by the MICROCOMPUTER or the like. Further, the overcurrent can be detected without the shunt resistor. Particularly, when the on/off control process for the semiconductor switch is realized by a hardware circuit, there is no need of using the MICROCOMPUTER. This feature brings about the packaging space reduction, and considerable reduction of the device cost.
In a case where, as in the power supply control device, the power supply control device further comprises overheat protector operating such that when the semiconductor switch is overheated, the overheat protector turns off the semiconductor switch, to thereby protect the semiconductor switch, when an imperfect short having some short resistance occurs, the semiconductor switch is repeatedly turned on and off by the controller, whereby the current is greatly varied. That is, in this case, the periodic heating of the semiconductor switch quickens the turning off of the semiconductor switch by the overheat protector. In particular, the power supply control device of the invention can process the abnormal current caused by the imperfect short (layer short), which cannot be processed by any way than the program processing by the MICROCOMPUTER or the like in the conventional device, by using only the hardware circuit contained in the power supply control device per se, or not using the control from an external device, such as MICROCOMPUTER. Therefore, the device circuit is simplified, and hence the device cost is reduced.
In the power supply control device, it is preferable that the semiconductor switch, the reference voltage generating device, the detector, the controller, and the second reference voltage generating device or the overheat protector are formed on one and the same chip. In the ninth power supply control device, the reference current generating device is preferably located outside the chip. By fabricating those means and switch into one chip in an integrating fashion, the device circuit construction is reduced in size, the packing space is reduced, and the device cost is reduced. In the invention, the current detection is based on the detection of a difference between the terminal-terminal voltage of the semiconductor switch and the reference voltage. With formation of the semiconductor switch and the second semiconductor switch on one and the same chip, the invention succeeds in eliminating (removing) common mode error factors which will appears in both the main semiconductor switch and the second semiconductor switch (reference voltage generator) to the almost same extend in the current detecting operation, i.e., adverse influences by power voltage drift and temperature drift, and uneven quality among lots. Additionally, with the feature that the reference current generating device is located outside the chip, the reference voltage (reference current) is made to be insensitive to temperature variation of the chip. Accordingly, high precision current detection is realized.
The power source terminal and the control signal input terminal of the semiconductor switch are connected to the power source terminal and the control signal input terminal of the second semiconductor switch of the reference voltage generating device, respectively. Further, the negative terminal of the second semiconductor switch is connected to the reference current generating device. With such an arrangement, judgement is made as to whether or not current flowing through the power supply path is within a normal range or an abnormal range by comparing a potential at the load terminal of the semiconductor switch with that at the negative terminal of the second semiconductor switch. Thus, the terminals of the semiconductor switch and the second semiconductor switch are commonized. Accordingly, those switches may easily be integrated onto one chip.
In the power supply control device, prohibiting device prohibits the controller from turning on and off the semiconductor switch for a fixed period after the semiconductor switch is turned on. Usually, when the load starts its operation, an inrush current usually flows into the power supply path. The inrush current is much greater than the current in a stationary state. If the controller executes the overcurrent control for the inrush current, some time is taken till the load is settled down into a stationary state. As a result, the response of the load per se delays sometimes. The invention solves this problem through the prohibiting control by the prohibiting device.
In the power supply control device, number-of-times controller adds up the number of times that the control means turns on and off the semiconductor switch, and when the number of on/off control times reaches a predetermined number of times, the number-of-times controller turns off the semiconductor switch. When an overcurrent by a short circuit is detected, the overheat protector quickly functions to make an overheat shut-off (turn off) of the semiconductor switch. In the case of the imperfect short, the semiconductor switch is repeatedly turned on and off. The resultant periodic heating of the semiconductor switch causes the overheat protector to function. Accordingly, it is estimated that time taken till the overheat shut-off is relatively long. In this connection, in the invention, when the number of on/off controls reaches a predetermined number of times, the semiconductor switch is turned off. Therefore, even when the imperfect short occurs, the turning-off of the semiconductor switch may be quickened to a desired turning-off time.